Navy Pier's largest fireworks display ever for America's 250th birthday was mostly hidden in fog Saturday night. The special Fourth of July show that began at 10 p.m. was longer than the typical summer show at Navy Pier, had nearly twice as many fireworks and covered double the area along the lakefront. But fog rolled in along the lake after Saturday's afternoon storms and obscured most of the spectacle. In a late-night statement, Navy Pier said it was "disappointed with the effect fog had on to